Commitee Members

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Publisher

University of Montana

Abstract

I investigated the processes of establishment of the cottonwood Populus trichocarpa T.&G. along the pristine, 5th order, wandering floodplain of the gravel-bed Flathead River, Montana, USA. The focus of my research was to determine the mechanisms explaining successful recruitment on recently formed, unvegetated bars, covering a range of spatial scales from within-bar (~ 1 to 10 m) to across-reach (~ 1 to 10 km). Results are based on field observations and measurements over three growth seasons (2005 through 2007), as well as outdoor and laboratory experiments.

At the within-bar scale, I used a paired-plot sampling design to compare closely-located sites with and without successful propagation, controlling for elevation and surface texture. I examined seedling response to soil texture in the root zone, thickness of the coarse surface layer of clean gravel, and depth to water. I found no effects of the vertical distance to water. On the other hand, the proportion of finer sediment (i.e., fine sand, very fine sand, and silt) within the matrix and the thickness of the coarse surface layer are controlling variables, that interact to explain both presence-absence and growth of seedlings. An evaporation experiment demonstrated that the coarse surface layer acts as mulch, strongly reducing evaporation and maintaining high soil moisture levels within the fines. I found that gravel bars are highly heterogeneous environments where the assumptions of horizontal and homogeneous water table and capillary fringe do not hold, and where cottonwood seedlings are essentially acting as facultative instead of obligate phreatophytes. Thus, some of the currently accepted concepts for cottonwood establishment do not apply in gravel-bed rivers.

Based on my observations of seeds and germinants in the drift, on experimental results confirming that cottonwood seeds germinate, establish and grow under water, and on comparisons of seedling establishment limits with flow profiles, I am proposing the River Seeding Concept of cottonwood establishment in gravel-bed rivers. This conceptual model posits that hydrochory is the primary establishment mechanism creating recruitment bands and patches of seedlings. Wind-dispersed seeds fall on the water, are incorporated into the drift, and are deposited by receding floodwaters along shallow shorelines, where underwater establishment occurs.